Sheep and lambs graze in a field near a rubbish dump. Photograph: STR New/Reuters

Massing in their millions, crucified and shredded on barbed wire fences, plastic bags have come to be dubbed "roadside daisies" in South Africa. Some now even mournfully refer to them as the country's national flower. Thousands of miles away in the metaphorical plughole at the heart of the Pacific Ocean, a spinning mass of plastic detritus, which includes countless carrier bags hanging limp in the water like jellyfish, revolves in perpetuity. And in China, which last year saw the closure of one of the world's largest plastic-bag factories, with the loss of 20,000 jobs, due to the government's concern about "white pollution", an estimated 300m carrier bags are still handed out to shoppers every day.

Plastic bags are one of the most recognisable symbols of our modern throwaway culture. In the decades since their introduction – the first plastic "baggies" for bread, sandwiches and fruit were introduced in the US in 1957 – their use has become ubiquitous across the planet. One million are handed out every minute, according to We Are What We Do, the not-for-profit group that was the driving force behind the Anya Hindmarch-designed "I'm Not A Plastic Bag" reusable carrier that briefly – and somewhat ironically – became a must-have accessory in 2007. It has long been the instinctive reflex of the shop assistant to place the items we buy into a plastic bag – and, equally, it has been our instinctive reflex to accept them. Very few of us ever questioned the logic or implications of such a mundane exchange. But in recent years, the unsightly and growing presence of these bags across our collective environment has led to a global movement to restrict their use – and, in some cases, calls for their outright ban.

According to reports yesterday, the Welsh Assembly is the latest government to consider outlawing the free distribution of plastic bags in shops. Jane Davidson, the Welsh environment minister, said that many shoppers were still failing "to embrace the environmental message" despite a raft of measures by supermarkets and other retailers to encourage us to use fewer of them. One solution now being given serious consideration by the Welsh authorities is the introduction of a 15p levy on every plastic or paper bag handed out to shoppers in the principality. Any revenue raised would be ring-fenced for local environmental projects, the minister said. (She also admitted that a small number of shoppers might be tempted to shop across the border in England to escape the tax.)

Evidence from across the world suggests that such a politically bold move would produce a dramatic drop in the number of bags being used each year in Wales. In 2002, Ireland introduced a 15 euro cents tax on each plastic bag – the so-called "plastax" – and within a few months a 90% reduction in the number of bags being used had been recorded. In total, the tax is thought to have led to a billion fewer bags being used each year in Ireland. The tax persuaded shoppers to bring their own reusable bags with them on shopping trips, or to request far fewer bags at the checkout.

But the scheme has had its critics. While it was true that the tax led to a dramatic drop in the number of bags being handed out in shops, it also triggered a 400% increase in the number of bin liners and black refuse bags being purchased. The tax also encouraged an increased reliance on paper bags which, according to a number of life-cycle analysis studies that have compared the environmental performance of various types of bags, require more energy to manufacture and release more greenhouse gases when degrading following their disposal. And while it is commonly accepted that plastic bags are a genuine blot on the landscape (and seascape), they only represent a tiny fraction of the waste stream by weight or by volume. For example, in the US they account for less than half a percent of domestic refuse.

The implication – expressed or otherwise – of such criticism is that we are either largely wasting our time pursuing such tactics in attempting to eradicate plastic bags, or that we are allowing ourselves to be distracted by what is, relatively speaking, a fairly minor environmental woe. James Lovelock, the climate scientist, has referred to the current obsession with plastic bags as "rearranging the deckchairs on the Titanic". Patting ourselves on the back about how few plastic bags we each now use allows us to ignore far more pressing environmental issues such as, say, climate change, overpopulation, rapid species extinction and the depletion of resources such as fresh water. Today's war on plastic bags is certainly worth fighting, but not if it is at the expense of these other concerns.

"It's the carbon content of what goes into your plastic bags, not the plastic bags themselves, that we should be worrying about," says Chris Goodall, the author of How to Live a Low Carbon Life, and Ten Technologies to Save the Planet. "This is 100% more important than, say, the amount of oil used to make one plastic bag. Plastic bags are a litter issue – yes, they certainly cause great damage to marine life – but they are frequently seen as a carbon issue. They are not. They are an easy target because they are one of the most visible environmental problems. But this doesn't make them the most important environmental issue. Many assume that recycling is the answer to the waste problem, rather than simply consuming less. It's not an easy message for many people to accept. Worrying about plastic bags also gives the illusion that small steps make a difference. This kind of radical change in thinking will take a generation."

Goodall says that the various efforts to reduce the use of plastic bags – be it through government legislation or the voluntary efforts (spurred on by high-profile campaigns by the likes of the Daily Mail) by supermarkets to reduce their customers' reliance on such bags – are invariably littered with unintended consequences. As has been seen in Ireland, plastic bag taxes often lead to a rise in the number of bin liners being purchased. "This plastic is much thicker and will prove to be a greater environmental hazard than thin plastic bags," he says.

The widespread belief that biodegradable "plastic" bags made from, say vegetable starch, are the panacea is also misguided, says Goodall. "I've still got a load of these bags sitting at the bottom of my allotment two years later."

And introducing a plastic bag ban or tax doesn't necessarily produce lasting results. In 2007, the Irish authorities were forced to increase their bag tax to 22 euro cents after the number of bags being used each year by every citizen rose from 21 to 31. (However, before the tax was introduced, the Irish were, on average, each using 328 bags a year.)

There are also growing rumbles of concern in San Francisco, which, in 2007, became the first city in the US to introduce a plastic bag ban. An investigation by the San Francisco Weekly earlier this year found that in the period since the ban was introduced there had actually been a slight rise in the number of plastic bags picked up off the city's streets.

All eyes are now on Seattle. In a week's time [18 August], its citizens will get to vote on whether to introduce a 20-cent levy on plastic bags. It represents one of the first occasions when an electorate has been asked if it wants such a levy rather than having it imposed on them by elected politicians. It's currently too close to call, but the lobbying for both sides of the argument has been intense. One local paper reported this week that the American Chemistry Council, the lobbying arm of the plastic industry that includes members such as Dow Chemical, ExxonMobil and some of the leading plastic-bag producers, had already spent almost $1.4m trying to defeat the bag tax, whereas environmental groups had raised about $80,000. As a result, some of the "yes" camp are now trying to dress up the battle as a vote against the influence of big oil.

But while Seattle's levy might not quite be in the bag just yet, there is already talk in some quarters about how an outright ban on all plastic bags is the ultimate goal. Haf Elgar, a campaigner for Friends of the Earth Cymru, welcomes the moves by the Welsh Assembly to consider a plastic bag levy, but believes the next logical step would be a complete ban. "Yes, we would support such a step," she says. "Charging, say, 15p for a bag is a great disincentive and a first step, but, ultimately, we all need to be bringing reusable bags with us to the shops."

Perhaps we need a dose of even more radical thinking: how about a tax on leaving home without a reusable bag? Think this is going too far? Earlier this year, a Beijing-based ecologist provoked a torrent of online abuse and ridicule when his suggestion – that tree planting be funded by a levy on individuals and businesses – was interpreted in media reports as a tax on breathing.

• This article was amended on 12 August 2009 to clarify that while the scientist, Jiang Youxu, proposed an environmental tax on businesses and carbon-producing city dwellers, he did not propose – as our original article said – a tax on breathing. This has been corrected.